Thanks for your instructive video. Ordinarily I will skip through most of the videos. In this instance I had a particular interest as my wife recently purchased a very similar shower unit to the one you have except the one, she bought is 3 sided and came without the door. This is to be a replacement to an old complete molded fiberglass unit It is larger and will require removal and replacement of a wall, so I have been putting off installing it due to some concerns about the sealing and fitment.
Your tip about the 1/4" blocks made the most sense. Maybe this winter. the guest bathroom will finally get the new shower installed now.